Victron BMV-712 showing different SOC to my BMS - which one do I trust?

Had this exact headache with my static van setup last summer. Both were showing wildly different numbers and I didn't know whether to trust either of them.

Short answer — trust the BMS for protection, trust the BMV-712 for actual usable SOC tracking once it's properly calibrated.

The BMS is really just there to stop things going catastrophically wrong (cell over/undervoltage, etc). Its SOC reading is often pretty rough, especially on cheaper units.

The BMV-712 does proper coulomb counting, so it tracks every amp going in and out. But it's only as good as your settings. Make sure you've got these dialled in correctly:

• Battery capacity (actual usable, not headline)
• Peukert exponent (matters more than people think)
• Charged voltage & tail current — this is what triggers a 100% sync, get it wrong and the drift just compounds over time

Mine was about 12% out until I tightened up the tail current setting. Also worth checking your shunt is wired so everything passes through it — easy to accidentally bypass it with a secondary load.

What BMS are you running? Some of the Daly units are notoriously optimistic with their SOC figures. Fogstar drift cells with a decent JK BMS is a whole different story in my experience.

Anyone else found a good way to get the two readings to converge, or do you just accept they'll never fully agree?

@Midge52 this took me ages to figure out on my motorhome build last year.

The penny dropped when I realised the BMV-712 drifts if it never gets a proper synchronisation point — it needs to see a genuine full charge to reset its baseline. Mine was reading 15% higher than reality because I'd been partial-charging for weeks on the road.

Fixed it by letting my Victron MPPT push the Fogstar lithiums to a proper absorption cycle, held until the tail current dropped right off. The BMV snapped back into agreement with the BMS almost immediately.

Now they sit within 2-3% of each other consistently. If yours are still wildly apart after a full charge cycle, worth checking your Peukert exponent and charge efficiency factor settings — the BMV defaults aren't tuned for lithium out of the box.

Right so this is exactly what happened with my shepherd's hut build last spring — drove me absolutely mental for weeks.

The bit nobody tells you upfront: the BMV-712 needs proper calibration time to learn your battery's actual behaviour. Mine was showing 94% while the BMS was screaming 78% and I genuinely didn't know which way was up.

What sorted it for me was nailing down the Peukert exponent and charge efficiency factor settings in the Victron app. They ship with defaults tuned for lead-acid essentially, and if you're running Fogstar or any other LiFePO4 cells you need to tweak those figures properly.

Once I'd done that and let it run through a few complete cycles, both readings converged to within about 3% of each other. Still use the BMS for protection decisions though — that's non-negotiable really.

The key detail nobody's mentioned yet is synchronisation drift — the BMV-712 relies on a clean 100% sync point (typically hitting absorption voltage with tail current below your configured threshold), and if that never quite happens due to partial cycling or a poorly-tuned tail current setting, the coulomb counting error compounds over days.

On my garden office system I found setting Charged voltage 0.2V below actual absorption and Tail current around 4% of capacity sorted it immediately. The BMV then syncs reliably every full cycle.

The BMS meanwhile is measuring actual cell voltages, so it's reacting to now rather than calculating history — which is why @Midge52 is right that the BMS wins on protection decisions.

Check your BMV's synchronisation log in VictronConnect — if it's not syncing at least weekly under normal use, your coulomb counter will wander considerably regardless of how accurate the shunt itself is.

@LochSpirit nailed the sync point issue — that's what was killing my readings for months on the motorhome before I figured it out.

One thing I'd add: your Fogstar or whatever cells you're running matter here too. If your BMS has slightly dodgy capacity figures programmed in, its SOC is basically fiction anyway. I've seen BMS units shipped with default 100Ah settings on 200Ah packs — absolute nonsense from the factory.

My actual workflow now:

• BMV-712 for day-to-day monitoring (it's brilliant once synced properly)
• BMS for protection cutoffs — non-negotiable
• Victron Cerbo GX ties it all together if you want proper logging

Calibrate both against a known reference charge cycle and they'll start agreeing within a few percent. Took me two full cycles to get mine settled after I reflashed the BMS firmware.